1. Field of the Invention
The present invention broadly relates to a vehicle-velocity calculating apparatus. More particularly, the invention relates to a vehicle-velocity calculating apparatus, for example, for obtaining velocity information required for performing positional calculation in a vehicle navigation system.
2. Description of the Related Art
In a conventional navigation system, the following type of vehicle-velocity calculating apparatus is available: an apparatus which uses a signal output from a vehicle-loaded velocity sensor so as to obtain velocity information required for performing positional calculation.
Another type of vehicle-velocity calculating apparatus is available in a known navigation system. An apparatus is provided with an acceleration sensor for detecting the acceleration of a vehicle traveling in a forward direction. The apparatus then integrates an output signal of the acceleration sensor with respect to time, thereby calculating the vehicle velocity.
However, signals cannot be readily extracted from the vehicle-loaded velocity sensor to the exterior, which inconveniently requires extra wiring work. Further, there is a difference in the method of outputting signals from the velocity sensor among the types of vehicles, and there may be some cases where signals cannot be extracted at all. Additionally, the following problem is encountered by a vehicle-velocity calculating apparatus which obtains the vehicle velocity by integrating a signal from the acceleration sensor with respect to time. If a vehicle travels downward, for example, on a slope, the integrated value representing the gravitational acceleration is disadvantageously output as the vehicle velocity.